The present invention relates to heat-resistant, acid-resistant, low-density open-cell porous materials, which are made from hollow microspheres, permeable for gas and liquids. Microspheres are cellulated glass hollow or solid microballoons, also known as spherical glass crystalline particles. Cenospheres are a particular class of hollow microspheres which are a component of fly ash obtained from the combustion of coal. The open-cell glass crystalline porous material of the invention is made from cenospheres, and has properties useful as porous matrices for immobilization of liquid radioactive waste, heat-resistant traps and filters, supports for catalysts, adsorbents and ion-exchangers.
Open-cell porous ceramic materials have been prepared in the prior art by means of foaming melts with the use of gas evolving additives, impregnation of ceramics on to a plastic network skeleton, and consolidation of different primary units (granules, fibers etc.). The meaning of xe2x80x9copen-cellxe2x80x9d porous materials used herein is porous materials with accessible internal voids composed of both voids between microspheres and voids inside the microspheres. Formed porous ceramic bodies differ considerably in their properties including texture (cellular or granular), open porosity, size of openings, and hydro- and aerodynamic resistance. For example, while the open porosity of cellular ceramics is up to 96 vol. %, the open porosity of granular materials is limited to about 40 vol. %. In spite of this, the porous structure of granular ceramics can be controlled more precisely by the shape and size of the primary units, especially in the case of microspherical particles. Other advantages of porous ceramics formed by microspheres are high compression strength and deformation ability.
The prior methods of forming porous ceramic bodies from microspheres were aimed predominantly at the creation of structural and insulating materials of small open or completely closed porosity, such as disclosed in U.S. Pat. Nos. 3,458,332, Re. 25,564, 4,016,229, 4,035,545 and U.S. Statutory Invention Registration (SIR) No. H200.
Heat-resistant porous structural materials of a 30-35% porosity comprising 50-75 vol. % of microspheres, 10-200 micrometers in diameter, of high-melting point oxides such as ZrO2, Al2O3, Y2O3, are disclosed in U.S. Pat. No. 4,035,545. The microspheres are sintered directly to each other so that the diameter of their contact amounts to 0.2-0.5 of the microsphere diameter. Composition of the material can incorporate 20-50 vol. % of a filler as metal, metal alloy, intermetallic compound, phenol-formaldehyde resin, polyvinyl alcohol, glass etc. The process steps for making the heat-resistant porous structural material include the plasma processing of the powdered high-melting point oxides to form microspheres, molding and isothermal sintering in an oxygen-gas-fired furnace at 1850-2100xc2x0 C. for 5-7 hours. The disadvantages of making such porous granular ceramic material are the high cost of initial components, high power consumption and complexity of the process.
U.S. Pat. No. 3,458,332 discloses the preparation of porous glass agglomerates of xe2x85x9 to xc2xd inch (3,175-12,700 micrometers) in diameter by sintering a mass of hollow glass microspheres with diameters of 5-5,000 micrometers and an alkalinity in the range from 0.103 to 0.192 milliequivalent per gram. According to the method, agglomerates of glass microspheres are formed by fusing the microspheres to each other at their points of contact by subjecting them to a temperature of 900-1100xc2x0 F. (482-593xc2x0 C.). No information was given about the porosity.
Closed-cell porous insulating materials have been prepared from hollow glass or ceramic microspheres. xe2x80x9cClosed-cellxe2x80x9d is intended to mean that porous materials have internal voids with closed walls which are not permeable for gas and liquids Porous lightweight ceramic bodies are disclosed in U.S. Pat. No. 3,888,691. These porous bodies have a comparatively high strength per unit of weight, obtained by mixing hollow glass spheres with refractory components, including refractory particles (lithium-aluminum silicate) and a binder (calcium aluminate cement and/or colloidal silica). The mixture is formed into a solid body and heated at below the softening temperature of the refractory particles and above the melting temperature of the glass within the spheres, in order to cause the glass to be drawn into the composition. As a result, closed spherical pores are formed in the ceramic body.
The porous material disclosed in U.S. Pat. No. 4,016,229 is a closed-cell ceramic foam material which can be prepared by heating hollow glass crystalline microspheres, recovered from fly ash from coal combustion (cenospheres), in the presence of air at 1350-1650xc2x0 C. for 0.25-1.5 hours. A coherent material having a bulk density of at least 0.50 g/cm3 is formed. The cenospheres may be used directly after recovery from fly ash but it is preferred to pretreat them by a decrepitation and/or separation procedure. The cenospheres are decrepitated by heating at a temperature of from about 315-540xc2x0 C. for 0.5-2 hours followed by separation in an organic liquid like heptane to obtain a fraction having a density of less than 0.35 g/cm3. To form the cenospheres into a predetermined shape a temporary organic binder such as gum arabic, or polyvinyl alcohol is used. Prior to firing, the decrepitated cenospheres can be admixed with 0.1-30 wt. % of an additive selected from the group consisting of transition metal and rare earth compounds, preferably transition metal and rare earth carbonates. The closed-pore ceramic foam may be used as a non-combustible insulation panel or structural member for a wide variety of applications.
A method of producing a structural insulating composite is disclosed in the U.S. Statutory Invention Registration H200. The method comprises (1) selecting hollow closed-cell ceramic beads having an outer diameter in the range of about 20-200 micrometers, a wall thickness of above about 2.0 micrometers, a softening temperature above about 800xc2x0 C. and a bulk density of about 0.3-0.5 g/cm3, (2) forming a mixture of the ceramic beads with a compatible binder composition with the weight ratio of beads: binder of 1:1-2, (3) removing entrained gas bubbles in the mixture and compacting the mixture under sintering conditions and pressure to provide the structural insulating composition. The sintering conditions include a temperature of above about 700xc2x0 C. but below the softening point of the microspheres. The final product obtained is characterized by a closely packed, bonded array of said beads with closed cells, useful as an insulating material at high temperatures.
Accordingly, an object of this invention is a method of producing an open-cell porous material, formed by cenospheres. Another object of the invention is a method of producing an open-cell glass crystalline porous material having open-cell porosity of up to 90 vol. %. A further object of the invention is a method of producing an open-cell glass crystalline porous material having a relatively low production cost. Another object of the invention is to produce an open-cell glass crystalline porous material having two types of openings, interglobular voids, i.e. voids between cenospheres, of 20-100 micrometers, and through-flow wall pores of 0.1-30 micrometers. An additional object of the invention is to produce an open-cell glass crystalline porous material having an open-cell porosity in the range of about 40 to about 90 vol. %, useful as a porous glass ceramic matrix for immobilization of liquid radioactive and other toxic waste, as a heat-resistant trap and filter, as a support for catalysts, an ion-exchanger and an adsorbent.
The material of high open-cell porosity which is characterized by two types of openings, interglobular voids and through-flow wall pores, is produced by separating cenospheres of fixed sizes and composition, molding the cenospheres and agglomerating the cenosphere array under sintering conditions. The separation steps include a required step of separation by density to remove the broken cenospheres and accessory particles such as unburned carbon, which are heavier than non-broken cenospheres. In addition the separation steps include one or more of the following steps, depending on the required parameters of the product: dry magnetic separation, separation by grain size, gravity concentration to group the cenospheres according to densities, and recovery of perforated and non-perforated cenospheres. To achieve the maximum open-cell porosity of 90%, the gravity concentration step (based on densities) is always performed. These steps, including the required step of removing broken cenospheres and other material can be performed in any order.
In one embodiment the cenospheres are separated into grain size groups, and into perforated and non-perforated cenospheres. The perforated cenospheres are selected and mixed with a wetting agent, such as water, and a binder, such as a liquid silicate glass, in a weight ratio of cenospheres:wetting agent:binder of about 1:(0.012-0.29):(0.15-0.30), followed by compaction of the obtained plastic mixture in a press form to reduce the mixture volume by 10-20%. The molded blocks are dried at 160xc2x0 C. for 2 hours and sintered for 0.5-1 hour at a temperature above 800xc2x0 C., e.g. at 850xc2x0 C., but below the softening temperature of the cenospheres. The softening temperature depends on the cenosphere composition, which depends upon the location from which the fly ash was obtained. For the non-magnetic cenospheres from the Novosibirskaya power plant, the softening temperature is about 1100xc2x0 C. Glasses are characterized by the softening temperature range. The low limit of this range is the softening temperature, and the high limit is the liquidity temperature, which is about 1400xc2x0 C. for non-magnetic cenospheres from the Novosibirskaya power plant. The non-perforated cenospheres are placed in a refractory mold of a predetermined shape, the mold is placed in a muffle and held at sintering temperature above 1000xc2x0 C. but below the liquidity temperature for 20-60 minutes. After sintering, the cenosphere agglomerate is additionally treated with acid reagents selected from the group consisting of 3-6 M hydrochloric acid; NH4Fxe2x80x94HFxe2x80x94H2O with content of Fxe2x88x92 about 15-30 gram-ions per liter at a molar ratio NH4F/HF of about 0.1-1.0; and NH4Fxe2x80x94HClxe2x80x94H2O with content of Fxe2x88x92 about 1-10 gram-ions per liter at a molar ratio Fxe2x88x92/Clxe2x88x92 of about 0.1-1.0. The cenospheres have a diameter in the range of 40-800 micrometers, preferably in the range of 50-400 micrometer, a softening temperature above about 1000xc2x0 C., a temperature of liquidity about 1400xc2x0 C., and bulk density above about 0.25 g/cm3. The resulting porous material is characterized by open-cell porosity in the range of about 40 to about 90 vol. %, interglobular openings in the range of 20-100 micrometers, through-flow wall pore size of 0.1-30 micrometers, an apparent density in the range of 0.3-0.6 g/cm3 and a compressive strength in the range of 1.2-3.5 MPa.
Cenospheres obtained from fly ash are a relatively cheap material of high quality obtained as a by-product in coal combustion at power plants. Cenospheres are characterized by spherical design, chemical and thermal stability, and high hydrostatic compressive strength of about 20-30 MPa at 50% destruction, and 10 MPa at 12% destruction. The composition of their shells includes predominantly Si and Al and a minor content of Fe, Mg, Ca, Na, K, and Ti. The chemical composition of cenospheres obtained from combustion of Kuznetskii coals (Russia) is presented in Table 1.
Cenospheres are chemically inert and are exempt from classification as a hazardous waste as determined by the United States Environmental Protection Agency. They are considered reclaimable under the Resource Conservation and Recovery Act (42 U.S.C. xc2xa7xc2xa76901-6992-15) and their reuse is labeled as environmentally sound.
Cenospheres are usually recovered from fly ash by flotation in water as a mixed material involving globules of different size, density, morphology and composition. To provide an open-cell porous material with predetermined parameters (open-cell porosity, compressive strength, apparent density, size of openings, composition), the cenospheres of fixed properties can be selected by one or more of the following steps, in any order: dry magnetic separation, grain size separation, density separation, for instance by gravity concentration by placing the cenospheres in organic liquids having a density less than water, and separation into perforated and non-perforated. Using the first three methods for separation of cenospheres from the Novosibirskaya power plant gives 24 products of different magnetizability (magnetic and non-magnetic products in a ratio about 1:20 by weight), sizes (xe2x88x92400+200, meaning less than the sieve hole size of 400 but greater than the sieve hole size of 200 micrometers, xe2x88x92200+160, xe2x88x92160+100, and xe2x88x92100+63 micrometers, for both magnetic and non-magnetic products), and bulk density (0.32, 0.43, 0.49 g/cm3 and 0.36, 0.45, 0.52 g/cm3 for non-magnetic and magnetic products, accordingly).
The chemical composition of the cenospheres is as follows. Data of chemical analysis indicate that the concentration of iron in magnetic products is 2-3 times greater than in non-magnetic products. The Mg and Ca content of magnetic cenospheres is also higher. On the contrary, the content of SiO2 and Al2O3 is lower than in non-magnetic products. As for other elements, the content of Na2O, K2O and TiO2 does not differ appreciably in magnetic and non-magnetic products. The following ranges of composition for magnetic and non-magnetic products accordingly are respectively as follows: SiO2xe2x80x9458.0-61.0 wt. % and 64.9-66.3 wt. %; Al2O3xe2x80x9418.2-20.4 wt. % and 20.1-21.1 wt. %; Fe2O3xe2x80x949.7-12.3 wt. % and 3.1-4.6 wt. %; MgOxe2x80x941.4-3.0 wt. % and 1.9-2.2 wt. %; CaOxe2x80x942.3-3.8 wt. % and 1.8-2.7 wt. %; Na2Oxe2x80x940.4-1.3 wt. % and 0.3-0.6 wt. %; K2Oxe2x80x941.8-2.7 wt. % and 1.9-2.9 wt. %; TiO2xe2x80x940.3-0.8 wt. % and 0.2-0.5 wt. %.
To provide an open-cell porosity of material based on using cenospheres, a cenosphere agglomerate is produced so that the hollow globules are sintered to each other at their points of contact either with or without a binder. To enhance an interglobular void of the sintered cenosphere array and to obtain openings of a predicted size, the cenospheres having diameters in a narrow range of values are preferable. The lightest fraction with an accessible interglobular void produced total open-cell porosity up to 90 vol. %, which is as high as porosity of the cellular porous bodies. It is also desirable to have through-flow pores in the cenosphere walls, which make the internal void of cenospheres accessible.
Perforated cenospheres which can be recovered by vacuum injection with water have been found in all fractions of cenospheres. Their total content in the cenospheres of Novosibirskaya power plant is 10-13 wt. %. The evidence from a scanning electron microscope (SEM) shows there are some cracks of 2-5 micrometers in width and through-flow pore holes of 10-30 micrometers in diameter on the cenosphere surface. Non-perforated cenospheres can be easily perforated with appropriate acid reagents, due to the irregular chemical and phase composition of the glass crystalline shell. The defects of the structure allow the cenospheres to be etched in local sites. The chemical composition of magnetic cenospheres of Kuznetskii coals (using size 160-100 micrometers) was measured, by non-destructive electron probe microanalysis, at different points on a single cenosphere shell, and the following ranges were found (in wt. %): SiO2xe2x80x9460-70; TiO2xe2x80x940.6-2.0; Al2O3xe2x80x9418-22; FeOxe2x80x942-6; CaO less than 1; MgOxe2x80x941-2; K2Oxe2x80x943-4.5; Na2Oxe2x80x940.3-0.5. The heterogeneities in glass composition were found to arise from fine inclusions of ore minerals corresponding to quartz, hematite, magnetite and mullite. Treatment of non-perforated cenospheres with hydrochloric acid is accompanied by leaching of soluble components of glass (Fe, K, Na) forming through-flow pores with openings of 0.1-0.3 micrometers corresponding to dimensions of leached crystallites. A more regular distribution of through-flow holes in the cenosphere shell was obtained by using mild reagents based on hydrogen fluoride. In this case the silica of the glass phase is subjected to the action of the reagent. By etching of the cenospheres with NH4Fxe2x80x94HFxe2x80x94H2O or NH4Fxe2x80x94HClxe2x80x94H2O, it became possible to obtain circular holes of 2-20 micrometers in diameter. Thus, variation of acid reagent produced through-flow holes in the cenosphere shell with openings in the range of 0.1-20 micrometers. Naturally perforated cenospheres recovered from initial material provides through-flow openings up to 30 micrometers.
In a preferred embodiment, the perforated cenospheres are agglomerated by mixing with water as the wetting agent, and a liquid silicate glass binder, in a weight ratio of cenospheres:wetting agent:binder of about 1:(0.012-0.29):(0.15-0.30) followed by compaction of the mixture, drying at 160xc2x0 C. for 2 hours and sintering at a temperature above 800xc2x0 C. but below the softening temperature, which in this example is 1100xc2x0 C., for 0.5-1 hour. The porous body obtained in such a manner has an open-cell porosity of from 55 to 75 vol. %. Compaction of the non-perforated cenospheres with the silicate binder under the same conditions produced a porous material of 40-50 vol. % open-cell porosity. This material is characterized by high stability to acids, excluding acid reagents based on hydrogen fluoride.
Non-perforated cenospheres can be agglomerated without any binder under sintering conditions which promote the perforation of the cenospheres. The resulting porous body has an enhanced open-cell porosity and is more stable to acids than material sintered with the binder. On heating of an array of non-perforated cenospheres, the glass walls start melting at a temperature of about 1000-1100xc2x0 C. and the softened walls stick to each other. It is believed that crystallization of the melt on cooling causes wall cracking and perforation because of the different coefficients of thermal expansion for crystalline and amorphous phases. The factors controlling an apparent density and open-cell porosity of the resulting porous material are temperature and time of sintering. For example, an open-cell porous material having the open-cell porosity of about 55-60 vol. % can be obtained from cenospheres of the Novosibirskaya power plant by sintering at 1100xc2x0 C. for 20-60 minutes. The further treatment of the sintered porous body with acid reagents provides an open-cell porosity of about 70-75 vol. % (using hydrochloric acid) and 85-90 vol. % (using NH4Fxe2x80x94HFxe2x80x94H2O or NH4Fxe2x80x94HClxe2x80x94H2O).